An ultrasonic probe is used for a fish finder, an ultrasonic diagnostic apparatus for a living body, etc. The ultrasonic probe is provided with a surface protection layer or an acoustic lens having acoustic protective functions when an ultrasonic beam is transmitted to a human body, etc.
Ultrasonic probes include a one-dimensional and two-dimensional array ultrasonic probe which arranges vibration elements one-dimensionally and two-dimensionally, respectively.
These days, a high-performance two-dimensional array probe capable of recognizing a volume of an object has been desired to be developed.
In a one-dimensional array ultrasonic probe with a one-dimensional array of vibration elements, a convex acoustic lens is formed in the outermost surface of the probe in order to focus ultrasonic waves transmitted from each vibration element. The lens is normally composed of rubber parts. Ultrasonic waves are focused basically in two directions of the one-dimensional probe. In a longitudinal direction, i.e., a direction along the one-dimensional array, focusing is done by delaying transmission timing of an ultrasonic wave from each element of the array. In a direction perpendicular to the longitudinal direction, focusing is done by using the convex acoustic lens. Acoustic impedance (AI=acoustic velocity×density) of the lens must be made closer to that of a human body (about 1.5 MRayls). However, it is advantageous to use materials for the lens having an acoustic velocity of 1000 m/s or less, which is lower than that of a human body, 1500 m/s, in order to obtain a feature of a convex lens. For this reason, materials having a rubber density of 1.5 g/cm3 or more and an acoustic velocity of 1000 m/s are used for the lens. On the other hand, ultrasonic waves transmitted from each vibration element can be focused electronically in a two-dimensional array ultrasonic probe with a two-dimensional array of vibration elements. Electronic focusing can be done in both X- and Y-directions of the array by delaying transmission timing of an ultrasonic wave from each vibration element. Therefore, it is not necessary to purposely provide materials of which acoustic velocity is lower than that of a human body for a surface layer of the array. It is essential to use materials of which acoustic velocity and density are as close as possible to those of a human body. For this reason, the surface layer is often called an “acoustic protection layer”. The surface layer of the two-dimensional probe is different from that of the one-dimensional probe mentioned above from a viewpoint of the characteristics. Consequently, it is required to develop new materials for an acoustic protection layer of the two-dimensional probe.
There exists butadiene rubber as a material of which acoustic impedance is near to that of a human body. There has been an example where the material was used as an acoustic lens of a one-dimensional ultrasonic probe, published on JP-A 1996-615 (Kokai).
However, a surface protection layer used in a two-dimensional probe is thinner and flatter than an acoustic lens used in a one-dimensional probe. It is necessary to optimize mechanical strengths of butadiene series rubber to use it for the surface protection layer. It is further required to obtain good results to a biocompatibility test. Biocompatibility tests include a irritation test (irritation or intracutaneous reactivity), sentilization test, and cytotoxicity test.